GrFP can express distance vector field req., program builder declares variable for it

src/gpu/glsl/GrGLSLFragmentShaderBuilder.h

Index: src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
diff --git a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
index 614b04f54b20a3467e4fb7e8d85afed3cb6ab623..9e6eec2004472d879b09cffaeb7ef16f2abe48d4 100644
--- a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
+++ b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
@@ -53,6 +53,10 @@ public:
         is in device space (e.g. 0,0 is the top left and pixel centers are at half-integers). */
     virtual const char* fragmentPosition() = 0;
 
+    /** Returns a variable name that represents a vector to the nearest edge of the shape, in source
+        space coordinates. */
+    virtual const char* distanceVectorName() const = 0;
+
     // TODO: remove this method.
     void declAppendf(const char* fmt, ...);
 
@@ -166,6 +170,7 @@ public:
     virtual SkString ensureFSCoords2D(const GrGLSLTransformedCoordsArray& coords,
                                       int index) override;
     const char* fragmentPosition() override;
+    const char* distanceVectorName() const override;
 
     // GrGLSLFPFragmentBuilder interface.
     void appendOffsetToSample(const char* sampleIdx, Coordinates) override;
@@ -235,6 +240,7 @@ private:
     bool       fHasSecondaryOutput;
     uint8_t    fUsedSampleOffsetArrays;
     bool       fHasInitializedSampleMask;
+    SkString   fDistanceVectorOutput;
 
 #ifdef SK_DEBUG
     // some state to verify shaders and effects are consistent, this is reset between effects by